The subject matter disclosed herein relates to a method for correcting or calibrating a pressure sensor used in measuring hydraulic fluid pressure in an all wheel drive system for a vehicle.
An all wheel drive system may be equipped, for example, in variety of vehicle types, such as cars, SUVs, trucks, cross-overs, and construction machinery. The all wheel drive system is a drive train system that may include a pair of differentials, with one differential located between the front drive half shafts and a second differential located between the rear drive half shafts. The differentials allow the left and right wheels attached to both the front and rear differentials to rotate at different speeds but still allow both wheels to transfer torque. A slip device in the all wheel drive system, such as an all wheel drive clutch, may allow the front and rear differentials to spin at different speeds, thus allowing all four wheels to rotate at different speeds from one another while allowing all four wheels to drive the vehicle, thereby providing improved traction. In some situations the all wheel drive system may be deactivated by, for example, disengaging the rear drive half shafts from the drive train system by disengaging the all wheel drive clutch. Alternatively, if the all wheel drive clutch is fully engaged, then the front and rear differentials may spin at the same speeds.
A linear hydraulic control device, relying upon hydraulic pressure, may be used to apply force to the all wheel drive clutch to selectively engage and disengage the clutch. This enables control over the distribution of torque to the rear differential and the rear drive half shafts. The all wheel drive clutch may be fully or partially engaged by varying the hydraulic fluid pressure to reach or exceed an engagement pressure. This varying degree of clutch engagement may be used to vary the torque distribution to the rear wheels (e.g., more engagement may transfer more torque, while less engagement may transfer less torque) by allowing the all wheel drive clutch to slip at lower pressures and to lock at higher pressures. The hydraulic fluid pressure may be controlled using a hydraulic fluid pump turned by an electric motor. The hydraulic fluid pressure may be measured using a hydraulic fluid pressure sensor, which may measure the pressure of the hydraulic fluid using an internal diaphragm, and may output the measured pressure as a voltage signal.
A variety of factors may cause the readings of such a hydraulic fluid pressure sensor to vary, reducing the accuracy of the sensor. These factors may include variations in the temperature of the hydraulic fluid, variations in ambient atmospheric pressure (e.g., due to altitude changes or pressure fronts), variations in manufacturing between pressure sensors, and wear of the pressure sensor over time. These variations can result in erroneous readings that may result in an all wheel drive controller reading a pressure value that is greater than a maximum pressure value or less than a minimum pressure value, resulting in all wheel drive being incorrectly or ineffectively engaged or disengaged, or an error flag being set by the controller and displayed on the instrument cluster of the vehicle. Furthermore, inaccurate readings may result in excess hydraulic pressure being applied through the system, putting additional stresses on the system which may cause components to fail or wear out more quickly. Conversely, insufficient pressure may cause the clutch to not engage or to slip excessively, which may also cause components to wear out more quickly. The errors due to the variation between pressure sensors during manufacturing may stay the same throughout the life of the sensor, while errors due to the wear of the pressure sensor and all wheel drive system may slowly vary over time.
The largest fluctuation in the pressure signal error may come from changes in the temperature of the hydraulic fluid and from the atmospheric pressure during a single driving cycle. As the all wheel drive system is activated, the hydraulic fluid may heat up, for example, due to the hydraulic fluid pump and clutch activity. The temperature and the atmospheric pressure may constantly change as the temperature of the hydraulic fluid increases or as the vehicle increases or decreases in altitude, and may also vary due to weather changes. These errors may cause the hydraulic fluid pressure sensor to output a voltage that is higher or lower than ideal voltage used to represent a particular pressure value.